In a solid state image pickup device, particularly in a CMOS (complementary metal oxide semiconductor) sensor, when many pixels are arranged in an array pattern (hereinafter, a region where many pixels are arranged in an array pattern is called a “pixel region”) and when a well potential of a common p-well is applied thereto from immediately outside a periphery of the pixel region, that is, from immediately outside a most external pixel, a time constant is different in a peripheral portion of the pixel region near the p-well contact and in a central portion of the pixel region far from the p-well contact. For example, if a drain voltage changes while power is being supplied by a whole-surface drain wiring, coupling between the whole-surface drain wiring and the p-well occurs, which leads to a change in a well potential of the p-well. As a result, the well potential changes over a long time period in the central portion of the pixel region, where a time constant is large, compared to the peripheral portion of the pixel region. Thus, a reference voltage momentarily differs in the central portion and the peripheral portion of the pixel region, which causes different outputs in the central portion and the peripheral portion of the pixel region. Accordingly, a phenomenon called shading may occur.
In order to prevent shading, a well contact may be provided in each pixel to stabilize a well potential. By providing a well contact in each pixel, a well potential can be stabilized even when a voltage of a drain terminal changes, the same reference voltage can be obtained in both the central portion and the peripheral portion of the pixel region, and shading can be prevented.
Hitherto, the well contact is formed by a typical CMOS process. Specifically, after forming a p-well which is common to all pixels, a p+-region having a higher p-type impurity concentration than the p-well is formed, an interlayer film is formed, etching is performed to form a hole in the interlayer film on the p+-region, a high-concentration contact ion implantation is performed on the hole, and then the hole is filled with an electrode. The p+-region is formed by using an ion implantation step for forming a source/drain of a P-channel transistor of a peripheral circuit region outside the pixel region (hereinafter referred to as a peripheral region), so that the p+-region has a high concentration of about 1×1020 cm−3 or more. In a recently-adopted CMOS process, a boron/fluorine compound, particularly BF2 (boron difluoride) is used as an ionic species in the ion implantation and contact ion implantation in order to satisfy a demand for a shallow junction.
As shown in FIG. 13, in a unit pixel, a photodiode 121 performing photoelectric conversion and a transfer transistor 131 transferring a photoelectric-converted signal charge are placed in an active region 113 in a p-well 112. Further, a well contact 114 is placed in the active region 113 in the p-well 112. Other transistors placed in the pixel are not shown in the figure. The well contact 114 may be placed in the active region 113 together with the photodiode, but in any case, the well contact 114 is placed on the p-well.
As described above, in order to provide a well contact in a pixel, a large amount of impurity such as B or BF2 need be introduced into a portion close to a photodiode or an active region, where the photodiode is placed, by using an ion implantation technique or the like. However, such high-concentration ion implantation often causes implantation damage (e.g., crystal defect) or mixing of impurity atoms, such as an undesired heavy metal. Since the crystal defect and heavy metal atoms may cause a dark current, the dark current occurs by providing a well contact in a pixel. If a large amount of dark current occurs in a pixel, that is observed as white spots in the dark (hereinafter referred to as white spots) on a captured image.
If a high-concentration p-type doped region is adjacent to the photodiode, a dark current occurs due to an intense electric field between the p-type doped region and an n-type layer of the photodiode, which may be a cause of white spots. These white spots result in degradation in an image quality, which arises a problem.
A problem to be solved is that, when a large amount of impurity need be introduced by an ion implantation technique or the like in order to provide a contact in a pixel, a dark current occurs due to an incidental crystal defect and mixing of an undesired impurity, which causes white spots.